Method for reducing nuclear medicine scanning time

ABSTRACT

A method for reducing the scanning time of SPECT broadly includes determining a physical characteristic of an object within a radiation field, calculating an optimal starting angle of at least two detectors based on the physical characteristic of the object and a trajectory of the at least two detectors such that the geometric efficiency of the detectors along the trajectory can be maximized, angularly displacing the detectors based on the calculated optimal starting angle, scanning the object within the field to detect one or more gamma photons emanating from the object, and preparing an image from the one or more detected gamma photons. Preferably, the physical characteristic of the object is its location within the field, but can include properties such as signal to noise ratio, shape of the object, etc.

FIELD OF THE INVENTION

The present invention generally relates to nuclear medicine, and systemsfor obtaining nuclear medicine images. In particular, the presentinvention relates to a method for reducing the scanning time of SinglePhoton Emission Computed Tomography (SPECT).

BACKGROUND OF THE INVENTION

Nuclear medicine is a unique medical specialty wherein radiation is usedto acquire images which show the function and anatomy of organs, bonesor tissues of the body. Radiopharmaceuticals are introduced into thebody, either by injection or ingestion, and are attracted to specificorgans, bones or tissues of interest. Such radiopharmaceuticals producegamma photon emissions which emanate from the body and are captured byscintillation crystals with which the photons interact to produceflashes of light or events. In SPECT, events are detected by one or morecollimated gamma photon detectors, also referred to as gamma cameras,which are typically rotated about a patient's body. Using theexperimental data that is collected, three-dimensional images of theorgans of the body, which have been taken up the radiopharmaceuticals,e.g. the heart, can be constructed.

While SPECT is a powerful tool in the clinician's toolbox, it suffersfrom at least one drawback—SPECT imaging can be time consuming whencompared with other types of imaging procedures. For example, whereas CTscanning procedures can take as little as a minute to complete, SPECTprocedures can take more than fifteen minutes to complete. This isproblematic because it can be difficult for a patient to remain stillfor such time periods, which can affect image quality. Similarly, somepatients may be unwilling to undergo scanning procedures that take suchlong periods of time.

The amount of time that it takes to scan a patient using SPECT can beattributed to a number of factors. Most significantly, however, is thefact that SPECT detectors include collimating devices that only allowgamma photons traveling along precise trajectories to interact with thedetectors. As a result, it can take time for a sufficient number ofgamma photons to interact with the detectors to produce an image. Otherfactors that can affect SPECT scanning time include, but are not limitedto, the distance between an object and a detector, the amount of tissuebetween an object and a detector, and angle of orientation of the SPECTdetectors with respect to the object being studied. For example, manySPECT detectors are oriented at −45° starting angles and travel alongelliptical trajectories centered about a patient's body. However, areasof interest, such as the human heart, are centrally located within thebody. As a result, the distances, angles of orientation, and trajectoryof the SPECT detectors are not optimized, or geometrically efficient,for performing such scans. Consequently, unnecessary time is expendedwhen scanning such objects using these types of systems.

What is needed then is a method for optimizing a starting angle of aSPECT detector to maximize the geometrical efficiency of the detectorsalong a trajectory.

SUMMARY OF THE INVENTION

The present invention broadly comprises a method for acquiring imageswithin a radiation field, preferably using a SPECT scanning system. Themethod broadly comprises determining a spatial location of an objectwithin a radiation field, calculating an optimal starting angle of atleast two detectors based on the spatial location of the object withinthe field such that the geometrical efficiency of the detectors along atrajectory can be maximized, angularly displacing the detectors based onthe calculated optimal starting angle, scanning the object within thefield to detect one or more gamma photons emanating from the object, andpreparing an image from the one or more detected gamma photons.

More specifically, the present invention establishes an objectivefunction that is the integral of the distance from an object, e.g., theheart of a patient, to a pair of detectors and optimizes it in the leastsquare sense. The objective function accounts for the offset of theobject, e.g., the heart, with respect to the detectors and optimizes thestarting angle of the detectors based on the trajectory. Because thedetectors are ultimately positioned closer to the object being scannedas a result of the modified starting angle, the geometric efficiency canbe increased such that a same number of counts can be acquired in ashorter scan time period.

In some embodiments of the invention a SPECT scanning system includes apair of detectors having an optimal starting angle that can be describedby D(φ)=D₁ ²(φ)+D₂ ²(φ) and f(α) = ∫_(α)^(α + θ^(∘))D(φ)𝕕φ,wherein D corresponds to detector distance from the object, θcorresponds to the period of the detector trajectory, φ corresponds todetector rotation angle during scanning, α corresponds to detectorstarting angle, wherein ƒ(α) achieves a minimum value.

In some embodiments and depending upon the configuration of thedetectors, θ can describe any period, but in some instances describesperiods of 90° or 180°. In some embodiments of the invention, each ofthe at least two detectors are disposed at a 90° angles with respect toone another. In some embodiments of the invention, each of the at leasttwo detectors are disposed at 180° angles with respect to one another.In some embodiments of the invention, each of the at least two detectorsare disposed at a 76° angles with respect to one another. In someembodiments of the invention, while the trajectory of the detectors canbe any trajectory, e.g., an ellipse, a circle, an arc, a line, or aportion or combination thereof, in most instances the trajectorycomprises an ellipse for accommodating the human body.

In some embodiments of the invention, the method includes determining aphysical characteristic of an object within a radiation field,calculating an optimal starting angle of at least two detectors based onthe physical characteristic of the object such that the geometricefficiency of the detectors along a trajectory can be maximized,angularly displacing the detectors based on the calculated optimalstarting angle, scanning the object within the field to detect one ormore gamma photons emanating from the object, and preparing an imagefrom the one or more detected gamma photons. Preferably, the physicalcharacteristic is spatial location of the object within the field, ordistance from the object to the detectors, but can include otherphysical properties of the object and/or of the object within the field,e.g., signal to noise ratio, shape of the object, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be more fully described by way of example withreference to the accompanying drawings in which:

FIG. 1 is a flow diagram illustrating a method according to the presentinvention;

FIG. 2 is a schematic illustration of a known SPECT scanning systemdepicting detectors 1 and 2 disposed with respect to one another at anangle of 90° and at a −45° starting angle; detectors 1 and 2 areillustrated as being at distances D₁ and D₂ from the patient's heart;

FIG. 3 is a schematic illustration depicting relative positions ofdetectors 1 and 2 of FIG. 2 during a typical SPECT scanning procedurealong an elliptical trajectory;

FIG. 4 illustrates a graph of a distance function, D(φ)=D₁ ²(φ)+D₂ ²(φ),along a scan trajectory from −45° to 90°; the shaded region illustratesthe distance function from −45° to 45°;

FIG. 5 illustrates an objective function,f(α) = ∫_(α)^(α + θ^(∘))D(φ)𝕕φ,of the trajectory, the light vertical line indicates detector startangle at which the minimum of the objective function is achieved;

FIG. 6 is a schematic illustration of a SPECT scanning system similar toFIG. 2, but depicting detectors 1 and 2 disposed at an optimal startingangle according to the present invention; detectors 1 and 2 areillustrated as being at distances D₁ and D₂ from the patient's heart;and,

FIG. 7 is schematic illustration, similar to FIG. 3, but depictingrelative positions of detectors 1 and 2 of FIG. 6 during a SPECTscanning procedure along an elliptical trajectory.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described and disclosed in greaterdetail. It is to be understood, however, that the disclosed embodimentsare merely exemplary of the invention and that the invention may beembodied in various and alternative forms. Therefore, specificstructural and/or functional details disclosed herein are not to beinterpreted as limiting the scope of the claims, but are merely providedas an example to teach one having ordinary skill in the art to make anduse the invention.

Referring now to the Figures; FIG. 2 illustrates a known SPECT scanningsystem wherein each of a pair of detectors, 1 and 2, form a 90° anglewith respect to one another. In the figure, it is seen that when apatient is centrally positioned between detectors 1 and 2, the patient'sheart is not centered between detectors 1 and 2, but is offset such thatdistances D₁ and D₂ are not equal to one another. Thus, as shown in FIG.3, as detectors 1 and 2 follow their elliptical trajectory about thepatient to scan the heart, the distances D₁ and D₂ vary as thetrajectory is traversed. Consequently, because the geometric efficiencyof the detectors is dependent upon the distance of the detectors to theobject, it is not maximized because the heart is offset.

As illustrated in FIGS. 1 and 4-7, the present invention addresses theabove-identified SPECT scanning problems by providing a method whereinthe starting angle of at least two SPECT detectors, is optimized basedon a physical characteristic of an object to be scanned, e.g., thespatial location of the object within the radiation field of the SPECTscanning system, the distance between the detectors and the object,signal to noise ratio, etc.

As illustrated in FIG. 1, an exemplary embodiment the invention broadlycomprises the steps of determining a spatial location of an objectwithin a radiation field, calculating an optimal starting angle of atleast two detectors based on the spatial location of the object suchthat the geometrical efficiency of the detectors along a trajectory canbe maximized, angularly displacing the detectors based on the calculatedoptimal starting angle, scanning the object within the field to detectone or more gamma photons emanating from the object, and preparing animage from the one or more detected gamma photons.

According to the method, the spatial position of the object can bedetermined using a preliminary image of the object to be scanned, forexample, by using an attenuation (μ) map of a patient. Alternatively,the spatial position of the object can be estimated using knownpre-existing data regarding a patient or information pertaining totypical circumstances, e.g., average positional location of the humanheart in typical subjects. Once the spatial position of the object isknown, the distance between the object and the detectors can bedetermined to determine the optimal starting angle.

Calculation of the optimal starting angle is based on the fact that thenumber of counts detected by a detector is inversely proportional to thesquare of the distance from the source of gamma photons to a detector(1). Consequently, a summation of the square of the distances from theobject to a detector can provide an estimate of the count rate and adistance function can be described for a pair of detectors by:D(φ)=D ₁ ²(φ)+D ₂ ²(φ)  (1)wherein φ is the detector rotation angle during a scan. During a scanperiod θ, the geometric efficiency of the detectors can be maximized ifalong the trajectory an objective function described by equation (2)achieves a minimum: $\begin{matrix}{{f(\alpha)} = {\int_{\alpha}^{\alpha + \theta}{{D(\varphi)}{\mathbb{d}\varphi}}}} & (2)\end{matrix}$where α is the starting angle of the detectors.

Using the minimum a calculated, the starting angle of the detectorassembly can, thus, be modified such that the scan time can be reducedwhile a like number of counts acquired.

The above-described algorithms, or other algorithms, for optimizing thestart angle of the detectors, can be in the form of a softwareapplication input into the electronics of the SPECT scanning system. Thespatial location of the object to be scanned can be automaticallydetermined using the preliminary image data, any preexisting data, orestimates. The optimal starting angle can be automatically calculatedbased on the spatial location of the object and the angular dispositionof the detectors with respect to one another. The detectors can then beautomatically adjusted based on the calculation. Thereafter, the objectmay be scanned and an image prepared therefrom as is known.

FIG. 4 is a graphical representation of the distance function, D(φ), fora pair of detectors disposed at 90° with respect to one another whereinthe scan period range is from −45° to 90°. In the figure, the scanperiod range from −45° to 45° is marked with shaded lines. As can beseen, the area under the curve, that is, the distance from the detectorsto the object, is smaller where the starting angle is larger than −45°but smaller than 0°. Consequently, the geometric efficiency of thedetectors can be increased by modifying the start angle of the detectorsto an angle between −45° and 0°.

FIG. 5 is a graphical representation illustrating the objective functionchange, f(α) = ∫_(α)^(α + 90^(∘))D(φ)𝕕φ,for a pair of detectors disposed at 90° with respect to one another. Asshown in the figure, when the starting angle of the detectors ismodified between −45° and 0°, the minimum of the objective function isachieved at −23°. Thus, based on the objective function, a startingangle of −23° optimizes the geographic efficiency of the detectors alongthe trajectory. When compared with starting angles of known SPECTscanning devices, typically −45°, a −23° starting angle providesapproximately 9% more counts over a 90° scan period. Consequently, wherethe −23° starting angle is utilized, the scan time is reduced whilesimultaneously acquiring the same number of counts.

Similarly, as illustrated in FIGS. 6 and 7, when the starting angle ofdetectors 1 and 2 is modified according to the present invention to anangle of approximately −23°, distances D₁ and D₂ are shorter whencompared with distances D₁ and D₂ at starting angle of −45°. Similarly,throughout several portions of the scan period, it is seen that thedistances of D₁ and D₂ at a −23° starting angle are less than if thestarting angle were −45°. Consequently, the geometric efficiency of thedetectors is increased at a −23° starting angle such that a shorter scantime can be obtained.

Furthermore, if attenuation of a patient's tissue is taken into account,the affects of a modified starting angle according to the invention aremore significant and favorable; attenuation of tissue exponentiallyreduces the number of counts due to increases in the gamma photon traveldistance within body. In other words, disposing the detectors at themodified angle reduces the amount of tissue that the gamma photons musttraverse and will increase the number of counts.

The experimental data, thus, illustrates that the scanning methodaccording to the present invention can reduce SPECT scanning times byapproximately 9% when compared with current procedures. Calculating andmodifying the starting angle of detectors, to an optimal angularposition based on the spatial location of the object, allows thedetectors to travel optimally along a scanning trajectory to receive arequired number of counts in less time. In sum, shorter scanning timescan serve to reduce stress that a patient may incur and render a patientmore conducive to undergoing SPECT procedures. Additionally, reductionsin scanning time increase image quality as reduced scanning timesdecrease the likelihood of patient movement during scanning procedures.Furthermore, when the geometric efficiency of the detectors is optimizedalong a trajectory, the difference in the number of counts between eachdetector can be reduced, which can improve image reconstruction whenusing filtered back projection.

It should be appreciated by those having skill in the art that while theexemplary embodiment described herein describes a pair of detectorsangularly disposed with respect to one another at an angle of 90°, theangular disposition of the detectors with respect to one another mayvary. For example, the detectors may be disposed at angles of 76°, 180°,etc. Also, while the trajectory of the detectors of the exemplaryembodiment is described as being elliptical, the present method may beapplied to optimize the geometric efficiency of detectors along anytrajectory. Furthermore, while the scan period of the exemplaryembodiment is described as 90°, the scan period is not so restricted.Also, while the present invention has been described in association withoptimizing geometric efficiencies of detectors based on spatiallocation, geometric efficiencies of the detectors can be optimized onthe basis of other physical characteristics of an object, for example,signal to noise ratio, shape of an object, etc.

It should be appreciated by those having ordinary skill in the art thatwhile the present invention has been illustrated and described in whatis deemed to be the preferred embodiments, various changes andmodifications may be made to the invention without departing from thespirit and scope of the invention. Therefore, it should be understoodthat the present invention is not limited to the particular embodimentsdisclosed herein.

1. A method of acquiring images from a radiation field comprising:determining a spatial location of an object within the field;calculating an optimal starting angle of at least two detectors based onthe spatial location of the object such that the geometrical efficiencyof the detectors along a trajectory can be maximized; angularlydisplacing the detectors based on the calculated optimal starting angle;scanning the object within the field to detect one or more gamma photonsemanating from the object; and, preparing an image from the one or moredetected gamma photons.
 2. The method of claim 1 wherein the detectorscomprise a pair of detectors configured for performing SPECT.
 3. Themethod of claim 2 wherein the optimal starting angle of the pair ofdetectors can be described by: $\begin{matrix}{{{{D(\varphi)} = {{D_{1}^{2}(\varphi)} + {D_{2\quad}^{2}(\varphi)}}};\quad{and}},} \\{{f(\alpha)} = {\int_{\alpha}^{\alpha + {\theta{^\circ}}}{{D(\varphi)}{\mathbb{d}\varphi}}}}\end{matrix}$ wherein, D corresponds to detector distance from theobject; θ corresponds to a period of the trajectory; φ corresponds todetector angle during scanning; and, α corresponds to detector startingangle; and, ƒ(α) achieves a minimum value.
 4. The method of claim 3wherein θ is 90°.
 5. The method of claim 4 wherein the each detector ofthe pair of detectors is disposed at a 90° angle with respect to oneanother.
 6. The method of claim 3 wherein θ is 180°.
 7. The method ofclaim 6 wherein each of the detectors of the pair of detectors isdisposed at a 180° angle with respect to one another.
 8. The method ofclaim 3 wherein each of the detectors of the pair of detectors isdisposed at a 76° angle with respect to one another.
 9. The method ofclaim 3 wherein the trajectory comprises an arcuate portion.
 10. Themethod of claim 3 wherein the trajectory comprises a line portion.
 11. Amethod of acquiring images from a radiation field comprising:determining a physical characteristic of an object within the field;calculating an optimal starting angle of at least two detectors based onthe physical characteristic of the object and a trajectory of the atleast two detectors such that the geometric efficiency of the detectorsalong the trajectory can be maximized; angularly displacing thedetectors based on the calculated optimal starting angle; scanning theobject within the field to detect one or more gamma photons emanatingfrom the object; and, preparing an image from the one or more detectedgamma photons.
 12. The method of claim 11 wherein the physicalcharacteristic is spatial location of the object within the field. 13.The method of claim 12 wherein the detectors comprise a pair ofdetectors configured for performing SPECT.
 14. The method of claim 13wherein the optimal starting angle of the pair of detectors can bedescribed by:D(φ)=D ₁ ²(φ)+D ₂ ²(φ); and, f(α) = ∫_(α)^(α + θ^(∘))D(φ)𝕕φ wherein, Dcorresponds to detector distance from the object; θ corresponds toperiod of the trajectory; φ corresponds to detector angle duringscanning; and, α corresponds to detector starting angle; and, ƒ(α)achieves a minimum value.
 15. The method of claim 14 wherein θ is 90°.16. The method of claim 15 wherein the pair of detectors are disposed ata 90° angle with respect to one another.
 17. The method of claim 14wherein θ is 180°.
 18. The method of claim 17 wherein the pair ofdetectors are disposed at a 180° angle with respect to one another. 19.The method of claim 14 wherein the pair of detectors are disposed at a76° angle with respect to one another.
 20. The method of claim 14wherein the trajectory comprises an arcuate portion.
 21. The method ofclaim 14 wherein the trajectory comprises a line portion.
 22. The methodof claim 11 wherein the physical characteristic is signal to noiseratio.
 23. A method of acquiring images with a SPECT scanning systemcomprising: determining a spatial location of an object to be scanned;calculating an optimal rotational starting angle of at least twodetectors based on the spatial location of the object and the trajectoryof the at least two detectors such that the geometrical efficiency ofthe detectors can be maximized along the trajectory; rotationallydisplacing the detectors based on the calculated optimal startingrotational angle; scanning the object within the field to detect one ormore gamma photons emanating from the object; and, preparing an imagefrom the one or more detected gamma photons.